1. Field of the Invention
The present invention relates to a semiconductor device, a fabricating method thereof, a substrate used to fabricate the semiconductor device, and a fabricating method thereof. More particularly, the present invention relates to a nitride semiconductor device, a fabricating method thereof, a substrate used to fabricate the semiconductor device, and a fabricating method thereof.
2. Description of the Related Art
A nitride semiconductor, particularly a semiconductor device using gallium nitride (GaN), is used as a power device operating at a high frequency and high output. In particular, a high electron mobility transistor (HEMT) is known as a semiconductor device suitable for performing amplification of high-frequency bandwidths in microwaves, quasi-millimeter waves, millimeter waves, etc. Technology enabling the HEMT to operate under high drain voltage is being developed to allow operation of the HEMT at a higher output.
The nitride semiconductor is a compound of nitrogen (N), that is a Group V element, and a Group III element. Examples thereof are gallium nitride (GaN), aluminum nitride (AlN), and indium nitride (InN). HEMTs that use a nitride semiconductor (hereinafter, nitride semiconductor HEMT) including a GaN layer and an AlGaN layer, composed of a GaN and AlN mixed crystal, are disclosed in Patent Reference 1 (Japanese Patent Application Publication No. 2004-200248, FIG. 1), Patent Reference 2 (Japanese Patent Application Publication No. 2003-229439, FIG. 1), and Patent Reference 3 (Japanese Patent Application Publication No. 2001-77353, FIG. 1).
In Patent Reference 1, a nitride semiconductor HEMT is disclosed, in which an AlGaN electron supply layer is formed on a GaN channel layer, and a gate electrode, a source electrode, and a drain electrode are formed on the AlGaN electron supply layer. Here, a semiconductor surface layer between the gate electrode and the drain electrode is an AlGaN layer.
In Patent Reference 2, a nitride semiconductor HEMT is disclosed, in which a GaN electron accumulation layer is formed on an AlGaN electron supply layer, and a gate electrode, a source electrode, and a drain electrode are formed on the GaN electron accumulation layer. Here, the semiconductor surface layer between the gate electrode and the drain electrode is a GaN layer.
In Patent Reference 3, a nitride semiconductor HEMT is disclosed, in which an AlGaN electron supply layer is formed on a GaN channel layer, a gate electrode is formed on the AlGaN electron supply layer, and a source electrode and a drain electrode are formed on the AlGaN electron supply layer, via a GaN contact layer. Here, the semiconductor surface layers between the gate electrode and the drain electrode are the GaN layer and the AlGaN layer.
However, in a conventional nitride semiconductor HEMT, a phenomena where drain current decreases (collapse phenomenon) becomes a problem when high drain voltage is applied. An explanation of the collapse phenomenon is as follows. FIG. 1 shows drain current (Ids)-drain voltage (Vds) characteristics when 10 V, 20 V, and 50 V are sequentially applied to the nitride semiconductor HEMT (conventional example) as the drain voltage. The drain current when a drain voltage of 20 V is applied decreases compared to the drain current when a drain voltage of 10 V is applied. Further, when a drain voltage of 50 V is applied, the drain current is approximately half of the drain current obtained when the drain voltage is 10 V. In a HEMT as such, a drain voltage of 20 V or more cannot be applied, and thus, high output operations are difficult.